It has been confirmed that, in an SiC power device, Si (silicon) escapes from an SiC surface on account of an activation heat treatment which is required in forming an impurity layer, so surface roughening appears. When the Si escape occurs, a carbon-rich layer is formed in the surface of an SiC substrate. Unless the carbon-rich layer is removed, drawbacks such as the increase of a leakage current arise in device characteristics. Therefore, a sacrifice oxidation step and a sacrifice oxidation film removal step are added in order to remove the carbon-rich layer.
The “sacrifice oxidation step” termed here is a thermal oxidation step. In performing the thermal oxidation step, a large difference in the rate of thermal oxidation is exhibited between in a region doped with an impurity and in a region not doped with the impurity. More specifically, an accelerated oxidation in which the thermal oxidation rate of the impurity-doped region becomes greater than that of the impurity-undoped region proceeds, so the thermal oxidation film of the doped region becomes thicker than that of the undoped region.
For this reason, when the sacrifice oxidation film has been removed by an HF treatment, there is formed a “constriction” in which, since the oxidation film of the impurity-doped region is thicker, it becomes concave as compared with the impurity-undoped region. The “constriction” becomes a factor for the appearance of a thickness nonuniformity at the formation of a gate oxide film, and it degrades the reliability of the gate oxide film. FIG. 9 is a sectional view of the vicinity of the gate oxide film 100 as shows this situation. As seen from the figure, a large thickness nonuniformity appears in the gate oxide film 100.
In this regard, a method for suppressing the surface roughening has been proposed in JP-A-2005-260267. Concretely, after an organic film pattern of photo-resist or the like is formed, impurity ions are implanted into the organic film pattern. Thereafter, a graphite film is formed by carbonizing the organic film, and high-temperature annealing is carried out using the graphite film as a mask.
According to such a method, owing to the masking with the graphite film into which the organic film has been carbonized, the surface roughening under the mask can be suppressed.
Also, a method for suppressing the surface roughening has been proposed in JP-A-2005-303010. Concretely, after a drift layer is epitaxially grown, Si is sublimated by a vacuum high-temperature heat treatment, thereby to form a uniform carbon layer. Activation annealing is performed by utilizing the carbon layer as a cap layer, whereby the impurity layer is activated. In case of employing the carbon layer in this manner, any impurity contained in an organic solvent as in the case of the organic type graphite film does not diffuse into the SiC substrate, and any influence can be prevented from being exerted on the device characteristics.
With the method stated in JP-A-2005-260267, however, the graphite film is formed of the organic film pattern employed for the ion implantation, so that a part opened for the ion implantation is not formed with the graphite film and is not protected.
In the open part not formed with the graphite film, accordingly, Si is sublimated by the high-temperature annealing, and the carbon-rich layer is formed to roughen the substrate surface. In the fabrication of the device, therefore, a sacrifice oxidation step must be added in order to remove this carbon-rich layer, and the “constriction” ascribable to the accelerated oxidation as stated above cannot be prevented.
Besides, in the case of the method stated in JP-A-2005-303010, after the carbon layer has been formed, an SiO2 film is formed on the carbon layer and is worked by photo-etching in order to selectively perform ion implantation, whereupon the ion implantation is performed.
However, when the SiO2 film is formed on the carbon layer being amorphous, the close adhesion of the film cannot be ensured. That is, when a microscopic pattern is worked, the SiO2 film serving as a mask material peels off, and it cannot function as the mask for the ion implantation, so that a desired device performance fails to be attained.
Besides, JP-A-2005-303010 states a method wherein a carbon layer is formed after an ion implantation step for forming an impurity layer has been executed. If, as stated before, the carbon layer is formed by epitaxially growing the drift layer and then directly sublimating the Si of the surface of the drift layer, this carbon layer can be formed by the series of steps. However, if the carbon layer is formed after the execution of the ion implantation step, steps for the ion implantation step must be separately performed.
Further, in the case where the carbon layer is formed after the execution of the ion implantation step, impurity regions have already been formed, and parts of disordered crystal structure have already appeared on that occasion. Therefore, when the step of forming the carbon layer is performed at the high temperatures of at least 1100° C. to at most 1400° C., eventually the parts become different in a carbonizing rate (namely, the sublimating rate of the Si) as compared with the SiC surface where a crystal structure is not disordered. Consequently, when the carbon layer is removed, the “constriction” appears in the same manner as in the sacrifice oxidation and the sacrifice oxidation film removal. As a result, the thickness nonuniformity of the gate oxide film cannot be prevented, and the reliability of the gate film cannot be ensured.
Thus, it is required for an SiC semiconductor device to suppress the thickness nonuniformity of an oxide film which is formed on an SiC surface.